Sentinel can target both trees and units if you adjust the targets allowed. While the spell effect itself does not work on units, the spell event still occurs.
I just tested Sentinel and tree's can't be targeted once they have a Sentinel. I can easily kill / recreate the tree, but it results in the awkward 'variation' change.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.